Against the background of economic growth, the energy sector must, on an international scale, keep pace with increasing demand for energy despite increasingly scarce resources whilst also responding to the dangers of climate change caused by the emission of greenhouse gases. Changes in the technologies that are used and the shift from conventional to renewable energy sources will obviously depend on the kilowatt hour price of the energy produced.
Nowadays, with on-going improvements being made, the efficiency of solar photovoltaic panels is roughly 15%. This efficiency is obviously still low and the price of the electricity produced is still higher than that of electricity produced using traditional methods—this often means that solar photovoltaic projects are only viable because they are subsidised. There are now hybrid solar receivers designed to reduce the cost of solar photovoltaic technology and to recover a portion of the heat that is to be distributed.
Document FR 2 727 790 relates to a hybrid photovoltaic and solar thermal module. A photovoltaic panel is used to produce electricity, and, at the same time, gas is fed underneath the photovoltaic panel in order to cool it. The heated gas releases its heat as it flows through a heat exchanger. The advantage of such a receiver is the fact that it makes it possible to extract and recover a portion of the heat that accumulates on the photovoltaic panel with the blower being powered by a portion of the electricity produced by the photovoltaic panel. However, the drawback is that the flow of gas used to extract the heat may not remove accumulated heat from the photovoltaic panel fast enough or sufficiently and overall efficiency is therefore not high and this limits the use of the system.
Document WO 2006/038508 describes a hybrid solar thermal/electricity generating system which uses the evaporator part of a heat pipe in order to recover the heat that accumulates on the solar photovoltaic panel. The heat pipe has a 2-plate structure in which there is a meandering hole through which a refrigerant fluid flows. The advantage of this system is that the high thermal flux transferred by the heat pipe makes it possible to keep the photovoltaic panel at a low temperature. Nevertheless, this system is the result of simply superposing a conventional photovoltaic panel and a heat pipe: the dissipation of heat from the photovoltaic panel is not optimised due to the high thermal resistance of the rear face of the photovoltaic panel which is formed by a layer of a copolymer of ethylene-vinyl acetate (referred to hereafter as “EVA”) and a co-laminated layer of polyvinyl fluoride (PVF) and polyethylene terephthalate marketed under the brand name “TEDLAR®”. In addition, the meandering hole in the heat pipe is provided in order to increase the heat exchange surface but this structure limits the circulation of the heat transfer fluid through the heat pipe. Such a system can only function at low solar intensity levels because when the solar intensity is high, the meandering hole prevents circulation of the heat transfer fluid towards the condenser and, ultimately, the heat pipe cannot operate correctly. Finally, with a 2-plate structure that has a meandering hole, the heat exchange surface of the heat pipe is independent of the surface of the photovoltaic cells and there is a risk that the temperature on the surface of the photovoltaic cells will not be homogeneous.
The current problem with hybrid solar thermal/electricity receivers is that there is a conflict between the average effective temperature of the photovoltaic cells and that of the thermal receiver. In fact, most photovoltaic cells operate more efficiently when their temperature is about the same as that of their surroundings, whereas a higher temperature is often needed in order to obtain good thermal efficiency. Because of the relatively low intensity of the sun's rays and the low efficiency of photovoltaic cells (approximately 15%), if one increases the average temperature of the photovoltaic cells in an attempt to recover high-temperature thermal energy, the peak power of the photovoltaic cells drops sharply (0.4%/° C.) and generating efficiency is even more adversely affected. Consequently, in order to avoid the problem of photovoltaic cells overheating, the most widely known solution is to cool photovoltaic cells by natural or artificial ventilation located underneath the photovoltaic panel with this heat then not being recovered but being released into the environment.
To sum up, hybrid solar photovoltaic electricity/heat receivers are juxtaposed devices that do not make it possible to optimise the simultaneous generation of electricity and heat using solar radiation.
In addition, another economic aspect plays an important role in solar energy products and the technology that is used. In conventional solar photovoltaic panels, the photovoltaic cells account for roughly 70% of the total cost of a panel. Using the silicon in the photovoltaic cell more efficiently helps reduce the total cost of the panel. Solar concentrator technology is a way of reducing the surface area of photovoltaic cells that is needed and hence the quantity of silicon that is required. However, concentration results in a very significant increase in the temperature of the photovoltaic cell, to the extent that the efficiency of the cells is only around 15% and 85% of the solar radiation is dissipated as heat. In addition, as stated above, a photovoltaic cell operates more efficiently when its temperature is approximately the same as the ambient temperature (except cells that use amorphous silicon). Consequently, heat must be dissipated in order to maintain the temperature of the photovoltaic cell and prevent this temperature from increasing to values that adversely affect the cell's efficiency.
To solve the problem of possible overheating in non-concentrating hybrid solar systems, the solution is often to bond a heat exchanger directly underneath the photovoltaic panel. However, the layers underneath the photovoltaic cells in a photovoltaic panel consist of materials that have a very high thermal resistance, such as EVA and TEDLAR®. These layers are components that are essential to a conventional solar photovoltaic panel and cannot be removed. Given the fact that the insolation intensity is relatively low, the heat that needs to be dissipated is not very great and the ventilation underneath a conventional photovoltaic panel is sufficient to control any overheating.
In contrast, in a concentrating solar photovoltaic system, the problem of photovoltaic cells overheating is crucial: the solution that is classically adopted is to combine a heat exchanger with a heat exchange surface that is much larger than that of the photovoltaic cells. The heat exchanger is placed underneath the photovoltaic cells with a heat transfer fluid that circulates inside it. Because the surface of the heat exchanger is very large and its heat exchange capacity per unit of surface area is not high, the temperature of the heat transfer fluid therefore remains relatively low.
Document WO 2004/042828 relates to such a system for air cooling a concentrating photovoltaic system down to ambient temperature. In this system, the photovoltaic cells are attached to a special-shaped heat pipe by an adhesive thermal conductivity layer, an intermediate metallic layer and an elastic plate or by an adhesive thermal conductivity layer, an intermediate metallic layer, and an adhesive thermal conductivity layer (or a layer of solder) and an elastic plate. A system like this has many disadvantages: firstly, the photovoltaic cells are not protected against the air and humidity and can therefore corrode; moreover, given that the solar radiation is concentrated, the electrical current produced is high and there is a risk of electrical conduction between the cells and the heat pipe because the intermediate layers are only thermal conductivity layers and metallic layers (electrically conductive); then, the intermediate layers are mechanically attached to the heat pipe by an elastic plate and this poses a mechanical problem on the photovoltaic cells because there is a risk of the photovoltaic cells being crushed by the differential force produced by a non-uniform temperature; finally, this system is only used to cool the cells using air with a considerable heat exchange surface and cannot produce a warm liquid coolant.